It is known to use finely divided, water-insoluble hydrous silicates as thickening and gelling agents for aqueous organic liquid preparations. These silicates are frequently available as colloidal clays of the montmorillonite and bentonite type.
Advantageously, the structure of the montmorillonite type clays is such that water and other polar molecules including certain organic molecules can enter between the unit layers of the lattice, thus causing the lattice to expand, i.e., a swelling effect occurs. The number of water layers readily absorbed between the structural layers of the montmorillonite depends on the type of lattice substitutions in the original structure. Possible substitutions include iron and lithium in place of aluminum in the octahedral sheet and aluminum to a limited extent for silicon in the tetrahedral sheet.
The net effect of thesesubstitutions is a negative charge on the structure which is always nearly of the same magnitude. This charge is balanced by cations held on the interlayer surfaces. These cations may be replaced with other suitable cations by appropriate treatment. The common exchangeable cations are Ca++, Na+, mg++ and H+. The type of exchangeable cation profoundly affects the physical properties of the mineral by influencing both the nature of the liquids held in the inter-layer space and the strength of the bonding force between the adjacent clay layers. Sodium bentonite which is described as a colloidal aluminum silicate containing montmorillonite has a similarly expandable structure.
These swellable or expandable clays normally hydrate in water. As a result of the hydration, a thickening effect is achieved upon dispersing the clay in the liquid system, especially under high shear conditions. Under low shear conditions, however, many types of colloidal clays are difficult to hydrate and surface active substances preferably are added to facilitate the process. Moreover, these thckeners are often high viscosity materials which are difficult to incorporate into viscous materials without at least initial dilution of the thickener or preferably, by addition of a surfactant.
In the past, therefore, a number of different types of polymers have been used for upgrading certain rheological and physical properties of clays and clay slurries for application as thickening agents in aqueous organic liquid systems. Thus, the viscosity of clay suspensions has been improved by adding polyethylene oxide to aqueous bentonite suspensions (Ger. Patent Publication No. 2,052,506) and by treating Western or natural sodium bentonite and subbentonite clays, i.e., calcium and magnesium variety of montmorillonite, with polymers containing carboxyl groups and higher polyethylene oxide (U.S. Pat. No. 3,687,846). The gel forming properties of montmorillonite clay have been improved by treatment with a complex of fatty amine and nonionic higher polymeric compound, such as polyoxyethylene lauryl ether and polyoxyethylene stearyl ether (U.S. Pat. No. 33,560,429). Other clay modifying polymers employed previously include polyoxyethylene fatty acid esters (U.S. Pat. No. 3,450,666). Furthermore, cellulosic thickeners, such as carboxymethylcellulose or its sodium salt, have been added to aqueous clay suspensions to improve such properties as viscosity, gel forming capacity, stability, ion exchange and salt compatibility (French Pat. No. 1,474,022).
Such modified clay systems have been used heretofore as thickening agents for various aqueous preparations and formulations. For instance, a paint thickening composition containing montmorillonite or benetonite, a nonionic surfactant of the polyoxyalkyl alcohol type and noncellulosic thickener is disclosed in U.S. Pat. No. 3,687,885 and a formulation of an ointment base with a mixture of clay and carboxymethylcellulose has been suggested [M.R. Baichwal et al., Indian J. Pharm., 28 (11), 296-300, 1966].
It is also desirable to improve certain properties of a thhickening agent. Firstly, it may be preferable to utilize as little thickeneer as possible to achieve the required results. Secondly, the thickening agent should be resistant to floccing in aqueous systems containing electrolytes; that is, the colloidal structure of the thickening agent should be stable to electrolytes which may cause coagulation and separation from the system. These electrolyes occur commonly as necessary components in various preparations such as industrial emulsions, cosmetic creams and lotions, paint and similar formulations. Thirdly, it is advantageous to have no syneresis when the thickening agent is aged, for example, for 30 days.